The present invention relates generally to liquid crystal display (LCD) devices, and more particularly to a system and method for asymmetrically actuating a liquid crystal display.
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled display areas called xe2x80x9cpixelsxe2x80x9d (the smallest segment of the display). Each individual pixel is adapted to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
An LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the area of the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be a full range of colors.
LCDs are actuated pixel-by-pixel, either one at a time or a plurality simultaneously. A voltage is applied to each pixel area by charging a capacitor formed in the pixel area. The liquid crystal responds to the voltage across the pixel by twisting and thereby transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions (different capacitance areas of a pixel being charged) or at different times depending upon the LCD illumination method. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated for only blue light to be transmitted while another for green light, and a third for red light. A greater number of different light levels available for each color results in a much greater number of possible color combinations. Colors may be combined from a red pixel, a green pixel and a blue pixel, each residing on a different LCD, to produce any desired combined pixel color. The three LCDs (red-green-blue or RGB) are optically aligned so that the resulting light from each of the corresponding RGB pixels produces one sharp color pixel for each of the pixels in the LCD pixel matrices. The array of programmed pixels constitutes one video frame. A sequence of video frames produces video images that may change over time (e.g., motion video).
LCD technology has reduced the size of displays from full screen sizes to minidisplays of less than 1.3 inches diagonal measurement, to microdisplays that require a magnification system. Microdisplays may be manufactured using semiconductor integrated circuit (IC) dynamic random access memory (DRAM) process technologies. The microdisplays consist of a silicon backplane, a cover glass and an intervening liquid crystal layer. The microdisplays are conventionally arranged as a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix. Each pixel responds to changes in voltage by changing its illumination characteristics. If electrical charge has been built up at a pixel, however, a change in voltage may not change the illumination characteristics as quickly as desired. This phenomenon this called xe2x80x9cstickingxe2x80x9d. It is desirable for pixels of a microdisplay to be actuated so that a series of images corresponding to received image information is viewable on the microdisplay without sticking or flicker.
The present invention reduces the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing a system and method for improving image quality of a liquid crystal display (LCD) by asymmetrically actuating pixels of the display.
In a system embodiment of the present invention, a liquid crystal display includes a transparent cover. A cover electrode is positioned adjacent to the cover. A liquid crystal layer is positioned parallel to the cover. On the opposite side of the liquid crystal layer from the cover electrode are pixel electrodes arranged in a matrix of rows and columns. The positions of the pixel electrodes define portions of the liquid crystal display referred to as pixels. A control circuit receives digital video information at an input. The digital video information is sufficient to determine states of individual pixels. The control circuit generates first and second actuation voltages for at least one pixel from the digital video information. First and second voltages for the cover electrode are also generated such that the absolute difference between the first cover electrode voltage and the first actuation voltage is different from the absolute difference between the second cover electrode voltage and the second actuation voltage, the differences are asymmetric. The first cover electrode and actuation voltages are applied to the cover electrode and pixel electrode respectively for a first period of time. The second cover electrode and actuation voltages are applied to the cover electrode and pixel electrode respectively for a second period of time. The ratio of the first period of time and the second period of time correspond to the asymmetric variance of the actuation voltages whereby charge is not built up on the microdisplay. In a more particular embodiment, the first and second cover electrode voltages are identical. In another more particular embodiment, the first actuation voltage is greater than the first cover electrode voltage and the second actuation voltage is less than the second cover electrode voltage. In another more particular embodiment, the control circuit generates a plurality of first actuation voltages as a positive frame and a plurality of second actuation voltages as a negative frame.
In a method embodiment of the present invention, a cover voltage is applied to the cover electrode in a liquid crystal display. Voltages greater than and less than the cover voltage are sequentially applied to pixel electrodes corresponding to pixels in a first region of pixels. The voltages sequentially applied to the first region have a first average voltage. Voltages greater than and less than the cover voltage are sequentially applied to pixel electrodes corresponding to pixels in a second region of pixels. The voltages sequentially applied to the second region have a second average voltage that is different from the first average voltage. An input is received that selects one of at least the first region and the second region. A new cover voltage is determined based at least in part on the average voltage of the region selected by the input. In a more particular embodiment of the invention, the regions include numbers and the input is received from a remote control having buttons with the indicated numbers.
A technical advantage of the present invention is that it controls the light level of pixels of a liquid crystal display. Another technical advantage of the present invention is that it can actuate pixels with different voltage magnitudes for different polarities to offset variations in electropositivity of the display materials and reduce flicker. Another advantage of the present invention is that it allows manual calibration of the cover electrode voltage after manufacture. Another technical advantage of the present invention is that is can reduce charge build up and resultant image sticking.
Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention. For example, one embodiment of the present invention may only provide the advantage of controlling the pixels of a liquid crystal display, while other embodiments may provide several of the specified and apparent advantages.